An investigation into the practical and theoretical aspects of hybrid cathodic protection

Galvanic anode technology has in recent years come to the fore as a cost-effective method of successfully mitigating the corrosion of reinforcing steel in concrete structures. Developments in the field of cathodic protection have included the introduction of a novel Hybrid anode system, which uses the same sacrificial anode to pass a short-term impressed current before being connected to the steel directly to provide a long-term galvanic current. Galvanic and hybrid technologies are often seen as less powerful solutions in the treatment of reinforcement corrosion, and the test methodologies which determine the efficacy of cathodic protection systems favour impressed current technologies. The work completed has investigated the application of traditional and novel corrosion assessment techniques to laboratory samples to assess the protection offered by the hybrid treatment methodology in both treatment phases. In addition, the response of both galvanic and hybrid anodes to environmental conditions has been recorded and assessed before being discussed in the context of steel protection criteria. Finally, an investigation is presented into the on-site deterioration of commercially pure titanium feeder wire installed as part of the hybrid anode system and potential solutions to the problem have been documented. The research undertaken found that the hybrid anode system is capable of protecting steel in challenging, aggressive environments. This was confirmed by steel corrosion rate and indicative steel potential measurements. The responsive behaviour investigation showed that the current output of galvanic and hybrid anodes responds rapidly to changes in the corrosion risk posed to the steel and that this has a direct effect on anode system lifetimes. An assessment of the polarisation-based protection criteria applied to steel in concrete has found that the standard inhibits the use of responsive behaviour, and that revisions which consider the present risk of steel corrosion by considering the corrosion current resulting from the relative aggressivity of the concrete environment would be more valid in their application. A cathodic protection system based on the concepts of pit re-alkalisation and pH maintenance can fully utilise galvanic anode responsive behaviour. It was discovered that the deterioration of commercially pure titanium feeder wire seen on site installations was due to anodising in the presence of chloride media which had the potential to lead to pitting corrosion. The pitting risk varied depending on the duration of the treatment and proximity to the installed anode. An anodically grown oxide delayed the onset of corrosion in aqueous KBr solution, but did not significantly increase the pitting potential.